Just my opinion but, I think to make civilization more enjoyable...Military units should cost population and growth rates and take far fewer turns to build. Ancient units should have very little to no production costs, As units become more complicated, production cost should go up some but not as much as they currently do. Ability to produce a unit should be dependent on city improvements like they are with boats and planes. It shouldn't take hundreds of years to make settlers. This would increase boarder skirmishes for good settlements and resources and make the balancing act of war, science, culture and wealth more engaging. Diplomacy should be more forgiving after several decades of peace.